Organic compounds, such as aromatic hydrocarbons, fluoresce when irradiated with electromagnetic radiation of certain wavelengths. The property of fluorescence, particularly in response to ultra-violet radiation, has been used for many years as the basis of a method of chemical analysis.
Fluorescence analysis has also been performed on cuttings or cores obtained during the drilling of wells to determine the presence of hydrocarbons in pore fluid. An example of such a technique can be found in U.S. Pat. No. 4,609,821. In techniques such as these, the cuttings or cores are cleaned to remove any drilling fluid products, which might otherwise interfere with the analysis, and the samples are crushed and extracted with a solvent, which is then analyzed. Alternatively, the sample is directly irradiated, and the fluorescence analyzed. While core-analysis techniques can provide reasonably accurate analysis of the core fluids, there are certain drawbacks. Cores are relatively expensive to obtain and must be returned to the surface for analysis. Also, since cores are only taken at specific locations, it is possible that a hydrocarbon-bearing formation can be missed. Cuttings are obtained continuously in drilling, but have the disadvantage that it is not possible to determine at the surface exactly where the cuttings originate downhole, making the identification of hydrocarbon-bearing formations difficult. Also, cuttings give no reliable indication of the extent of any hydrocarbon-bearing formations.
It has been proposed to use fluorescence of hydrocarbons for downhole evaluation of pore fluids with a wireline logging tool. U.S. Pat. No. 2,206,922 discloses a wireline tool which is lowered into a well and uses fluorescence in response to ultra-violet radiation to detect hydrocarbons. In one application, it is proposed to use the '922 tool to detect the presence of oil in drilling fluid as an indication of oil-bearing strata. In another application, it is proposed to bail drilling fluid from the well, and examine the borehole wall for oil.
Unfortunately, as would be readily apparent to a person skilled in the art, neither application is feasible in practice. In the first case, the presence of any fluorescent materials (such as oil) which are added to the drilling fluid masks fluorescence from any oil entering the borehole from the formation. (Only small amounts of oil will enter a borehole due to mud overpressure.) Also, oil entering the borehole from the formation becomes dispersed during drilling or migrates up the well due to its differing density to the drilling fluid. Therefore, this approach will not allow accurate identification of oil-bearing strata. In the second case, the option of removing drilling fluid from the well is not usually available and, in any case, even such fluid removal would not remove mudcake from the borehole wall, which itself will mask the underlying formations.
Thus, the '922 patent fails to disclose any method or apparatus which might conceivably work in an oil well drilled using current technology. In particular, there is no disclosure of a technique which could be used to reliably log the length of the well with any reasonable expectation of being able to identify hydrocarbon-bearing formations.
U.S. Pat. No. 2,346,481 proposes a fluorescence logging tool which can be lowered into a well and logged to identify oil-bearing strata. In this case, the tool includes an ultra-violet light source. A transparent bar of quartz projects from the light source and is maintained in contact with the borehole wall. A further quartz bar is provided to direct the fluorescence to a film recorder as the tool is logged through the borehole. While the '481 patent appears to address some of the problems associated with the approach described in the earlier '922, the '481 system would still fail to function as described or intended--and persons skilled in the art would so recognize. In order to examine the borehole wall using the '481 device, it would be necessary for the quartz rods to penetrate the mudcake surrounding the wall. However, the force which must be applied to the quartz bar to ensure such contact would cause the bars to fail mechanically. Moreover, even in the absence of significant mudcake, the mere action of logging the '481 tool through a borehole would likely cause mechanical failure of the quartz bars. Therefore, the approach set forth in U.S. Pat. No. 2,236,481 is also inoperable.
U.S. Pat. No. 2,334,475 also discloses a fluorescence logging tool for detecting petroleum in situ. The tool includes an ultraviolet source which illuminates the rock through a quartz window and detects fluorescence through the same window. The window is suggested as being glass or quartz and, in one case, is housed in a mounting which includes portions for cutting through the mudcake on the borehole wall. It is suggested that quartz be selected as a material which is sufficiently resistant to withstand the friction of the borehole wall during the logging application and that the window project from its housing to come into direct contact with the borehole wall. However, as a person skilled in the art would recognize, quartz is not sufficiently strong to withstand the forces generated during logging, and will fail by fracturing. Thus, the '475 device also is inoperable and, in all likelihood, would not withstand more than a few feet of logging before tool failure.
Additionally, beyond the initial act of creating a fluorescence log, there remains the question of how to extract useful information from the log. The prior art merely suggests that fluorescence data can be used to ascertain the presence or absence of hydrocarbons leaching from the formations into the borehole. However, interpreting a fluorescence log to ascertain the presence of oil (or other geological feature(s) of interest) is a non-trivial problem.
U.S. Pat. No. 5,656,810, incorporated herein by reference, describes the use of emission spectroscopy in the evaluation of crude oil samples (in the laboratory, not downhole).
Besides crude oils, applicants have discovered that a number of minerals fluoresce. For instance, many limestones and some oil shales also fluoresce and, as applicants have discovered, such mineral fluorescence is primarily attributable to the presence of hydrocarbons in the rock.
Despite these similarities between oil and mineral fluorescence, experienced mud loggers can generally distinguish one from the other--based upon color, shape of the spot, and, if still in doubt, by solvent extraction. Unfortunately, none of these techniques is readily applicable to the downhole environment, especially the case of a continuous wireline log.